This invention relates generally to drivers for electronic display devices and, in particular, to a display driver for a vacuum fluorescent display. The invention finds application in vehicle electronic systems, such as display mirrors, as well as non-automotive applications. Advantageously, the present invention facilitates the use of vacuum fluorescent displays in battery operated devices.
A vacuum fluorescent display is similar in structure to a triode vacuum tube. The display includes a filament which must be kept hot in order to emit electrons and a grid which controls the flow of electrons from the filament to one or more phosphorous-coated segments. When a voltage differential of sufficient magnitude exists between the filament and a particular segment, the segment phosphors thereby emitting light. The grid typically determines whether an entire digit is ON or OFF and may also provide an intensity control in order to control the intensity of the digit. Alternatively, the intensity level of the digit may be controlled by Pulse-Width Modulation (PWM) of the signal turning on particular segments of the display.
One particular type of driver 10 for a vacuum display 12 (one digit only of which is shown in FIG. 1) requires a power supply 14 which produces both a positive polarity voltage, such as +5 volts, and an opposite polarity voltage, such as -7 volts. A microcomputer 16 having a built-in driver has output lines 18 connected directly with individual segments 20a-20g of display 12. Microcomputer 16 additionally produces an output 22 in order to operate grid 24. Filament 26 of display 12 is supplied with approximately 1 volt from the negative terminal of power supply 14. This is accomplished by supplying -7 volts to one terminal of the filament and -8 volts to the opposite terminal for a voltage drop of 1 volt. As long as grid 24 is maintained at +5 volts, any segment 20a-20g driven to +5 volts by microcomputer 16 is lit because of approximately a 12-volt differential between each such segment and filament 26. Non-lit segments are driven to -7 volts which produces no voltage differential between that segment and the filament. When grid 24 is at +5 volts, the digit is operable. When microcomputer 16 drives grid 24 to -7 volts, the entire digit is dark.
The difficulty with display driver 10 is that it requires a complex bipolar (3 output) power supply. Many systems, such as vehicular electrical supply systems, supply power at a single polarity typically between +9 volts DC and +18 volts DC (+12 volts DC nominal). Accordingly, circuitry to convert a unipolar power source to bipolar power supply adds extra cost and complexity to the system.
An alternative prior art vacuum fluorescent display driver circuit 30 is provided which is compatible with the unipolar nature of a vehicle power source. Display driver circuit 30 includes a microcomputer 32 which is operable from a +5 volt source and a separate driver circuit 34 which receives a coded output 36 from microcomputer 32 and decodes the output to provide appropriate signals via output lines 38 to display 12. Driver circuit 34 is operated from +12 volts which is of the same polarity as the source for microcomputer 32. Accordingly, both microcomputer 32 and display driver 30 are compatible with vehicular electrical supply systems. In order to produce the necessary voltage differentials to illuminate the various segments, driver circuit 34 switches output lines 38 to +12 volts in order to light a segment and to zero volts in order to cause a segment to remain dark. Grid 24 is supplied with +12 volts in order to switch the digit ON and at zero volts in order to turn the digit OFF. Driver circuit 34 supplies a +12 volt output which is used to heat filament 26. In order to supply the appropriate power to the filament, it is necessary to drop the 12 volts to 1 volt using a resistor R in series with filament 26. The other terminal of filament 26 is connected to ground. Therefore, filament 26 is close to zero volts. When a particular segment is supplied with +12 volts, a 12-volt differential exists between that segment and the filament in order to light the segment.
Although display driver 30 is compatible with vehicular supply voltages, it is not without its difficulties. A separate driver circuit is required in order to convert output voltages of the microcomputer to voltage levels sufficient to operate the vacuum display device. This adds cost and complexity to the circuit. Also, the necessity for a resistor to drop the supply voltage for the filament from +12 volts to +1 volt dissipates a significant amount of power resulting in a significant power consumption for the display driver. For example, display driver 30 requires approximately 150 milliamps at 12 volts DC.